Applications for non-linear optics include optical switching and electrooptic processing devices useful in, for example, telecommunications. Electrooptic devices generally provide electrical control of the direction, phase, frequency and/or amplitude of light wave signals. Materials fabricated with non-linear optical properties can be integrated with electronic devices on a semiconductor chip, laser or other solid-state devices to form monolithic optical or electrooptical semiconductor devices.
When a local electric field E induces polarization P in a molecule, then Equation 1 illustrates the polarization when E is expanded in powers of the electric field. EQU P=.alpha.E+.beta.E.sup.2 +.gamma.E.sup.3 +. . . (1)
The first term .alpha. is the linear polarization and is the origin of the refractive index if the field E is associated with an electromagnetic wave in the optical frequency range. When an electromagnetic field interacts with a molecular medium, the field polarizes the molecules. These polarized molecules act as oscillating dipoles broadcasting electromagnetic radiation. For a non-linear molecule the induced polarization is a non-linear function of the applied field so that the first non-linear term .gamma. of Equation 1 makes a significant contribution to the inducedfrequency components.
Non-centrosymetric crystals can exhibit harmonic generation frequency doubling (2.omega.), sometimes also called "second harmonic generation" or "SHG". This is the conversion of coherent light of frequency .omega. into light of frequency 2.omega.. Another example of a second-order non-linear optical effect is the Pockels effect where a DC field is applied to a medium through which an optical wave propagates. Again, this effect arises though .beta..
Molecules containing conjugated .pi. electronic systems with charge asymmetry exhibit very large values of .beta.. U.S. Pat. No. 4,859,876, discussed below, describes many examples of suitable non-centrosymetric organic molecules containing conjugated .pi. electron systems useful as dopants in materials having non-linear optic properties. Examples of other suitable polymers and dopants are given in U.S. Pat. No. 4,865,430, issued Sept. 12, 1989, inventors DeMartino et al.
As further background, a thermodynamic model and summaries of methods for measuring non-linear optical responses are discussed in the publications by Williams, Angew. Chem. Int. Ed. EngI. 23, pp. 690-703 (1984) and Singer et al. Appl. Phys. Lett., 49, pp. 248-250 (1986).
In order to produce observable second harmonic generation, a system must not possess a center of symmetry. One way to remove the center of symmetry is to align molecules with a permanent dipole moment by imposing a DC electric field. That is, for systems that consist of dopant molecules with permanent electric dipole moments in a polymer matrix, an applied electric field may be utilized to align the dopant molecules, thereby creating a noncentrosymmetric medium. When the alignment energy resulting from the interaction of the electric field is small compared to the thermal energy, an oriented gas model may be employed to predict the second harmonic generation coefficient. If the molecules are constrained in this position, then the second harmonic generation coefficient (often represented as X.sup.(2)) can be predicted through the simplified formula of Equation 2 for low field strengths with respect to the thermal energy where the alignment of molecules in an electric field is found to vary approximately with the strength of the electric field and the inverse of temperature: ##EQU1## Where N represents the number of dopant molecules, .beta. can be viewed as the intrinsic molecular hyperpolarizability of dopant (its definition being given by Equation 1), .mu. is the dopant permanent dipole moment, E is the applied electric field, and kT is the Boltzmann energy. As is seen from this approximate thermodynamic prediction, one wants N as large as possible, .beta. as large as possible, E as large as possible, but T as low as possible. The electric field is, however, limited by dielectric breakdown of the polymer and N is limited by phase separation and solubility considerations.
U.S. Pat. No. 4,859,876, issued Aug. 22, 1989, inventors Dirk et al., describe an electrooptical directional coupler or switch with channel wave guides. These wave guides are comprised of an optical quality glassy polymer host doped with a directionally ordered array of non-centrosymetric polar organic molecules which exhibit second order optical susceptibility in response to an applied field. This material is formed by means of spin coating as a 1.mu. thick film on a substrate such as silicon wafers. The film is formed of a non-crystalline, glassy polymer in which a dopant, such as red azo dye, is present as non-centrosymetric organic molecules dissolved in the polymer. This film is raised about 5.degree.-40.degree. C. above the glass-rubber transition temperature of the polymer by slow heating and a poling voltage is then applied across the film. The film is then cooled followed by an annealing step. The poling voltage is then removed.
However, the use of heat to reach a glass transition temperature while aligning dopant has disadvantages in that there is a loss of ordering of the dopant due to the elevation of temperature used to bring about the rubbery state of the polymeric matrix. Also, as the temperature is increased, the strength of the field needed to obtain a given degree of orientation of the dopant is increased. The temperatures required for a thermally induced glass transition of the polymeric materials, such as described by U.S. Pat. No. 4,859,876, are low enough that significant dopant orientation is possible. However, the use of polymers possessing (relatively) low Tg's shortens the lifetime during which the material retains SHG. It has been shown that there is significant loss of the SHG signal with time due to sub Tg relaxations of the polymer. The relaxation problem is magnified in that the guest chromophores are expected to lower the Tg of the supporting matrix. This plastization process results in the further attenuation of SHG intensities.
It has been shown that increasing the Tg of the polymer matrix results in a more persistent SHG signal. As the temperature needed to bring about the glass transition is increased, the strength of the field needed to obtain a given degree of orientation of the chromophore is increased. However, the strength of the electric field applied to orient the chromophore is limited by the dielectric breakdown strength of the material. Thus, for polymers with high Tg it is not always feasible to use a thermally induced glass transition for alignment purposes.
Japanese laid-open patent application No. 44427/89, laid-open on Feb. 16, 1989, assigned to Fujitsu Limited, describes a high molecular weight organic non-linear optical material formed by placing a solution in which a dopant and polymer have been dissolved with an is evaporated while applying a high voltage between the electrodes and the solution is solidified. The organic solvent exemplified is dichloromethane. This approach avoids the problems associated with using a thermally induced glass transition, but use of an organic liquid solvent creates its own problems in that it is very difficult to remove all traces of such solvents without resorting to elevated temperatures. Retention of even minute amounts of the solvent leads to microdomains in which the polymer is plasticized. Further, the polymer selection is limited by the liquid solvent one can use.
Accordingly, there remains a need for better methods of fabricating polymers with non-linear optical properties for applications such as in electrooptic devices.